1. Field of the Invention
The invention relates to a printing device used in an image forming apparatus including a printer, a copying machine, a plotter, and a facsimile, and to the image forming apparatus. The printing device performs printing by causing a pressing member to come into pressure contact with a fixing member to form a nip between the fixing member and the pressing member, and by causing a recording material to pass through the nip to fix a toner image on the recording material.
2. Description of the Related Art
According to a conventional electrophotographic printing device, an electrostatic latent image corresponding to an original image is formed first on a photosensitive element. Then, a visible toner image formed from the electrostatic latent image is transferred onto a recording material, and the transferred toner image is fixed by a fixing unit on the recording material to obtain a permanent toner image.
A fixing unit used in the printing device functions to obtain a permanent toner image by pressing and heating an unfixed toner image formed on a surface of a recording material. Fixing temperature control shown in FIG. 11 is conventionally conducted during the pressing and heating.
The fixing temperature control uses a paper type (coated paper or uncoated paper) and a paper thickness as parameters. An actual situation is that an operator sets a paper type and a paper thickness to select a fixing temperature set in advance for the data thereby determined, and the selected fixing temperature is corrected or changed. Environmental temperatures (temperatures inside and outside the device) are generally acquired by a sensor placed in the device in addition to the aforementioned paper type and the paper thickness. Then, in order to respond to the environment in which the printing device is placed (actual temperature), a reference fixing temperature (temporary value) is corrected to reach a controlled fixing temperature (determined value) as a target value. More specifically, in a relatively high-temperature environment during summer, a fixing temperature is lowered by correction for high temperature (temperature lowering correction for the fixing temperature to fall within an allowable temperature range). In a relatively low-temperature environment during winter, a fixing temperature is raised by correction for low temperature (temperature raising correction for the fixing temperature to fall within an allowable temperature range). As described above, the printing device is operated such that a permanent toner image is formed on a recording material by using a fixing unit that operates while a fixing temperature is maintained to fall within an allowable temperature range.
An allowable temperature range within which favorable fixing is realized is predetermined as a favorable range of fixing between cold offset and hot offset. The cold offset is a phenomenon where toner sticks to a fixing roller or a belt, or toner is not fixed on a recording material sufficiently due to a low fixing temperature. The hot offset is a phenomenon where adhesion between toners is reduced due to a too high fixing temperature to cause reattachment of the toners to a fixing roller or a belt.
Japanese Patent Application Laid-open No. 2004-184621 discloses an image forming apparatus with a fixing unit. The fixing unit includes an element 3 to be heated by heating elements 2A and 2B, and a fixing film 4 heated by the element 3 to be heated. The fixing film 4 and a pressing roller 5 are caused to come into contact with the element 3 to be heated to form a nip N therebetween for fixing a recording material. The heating elements 2A and 2B on the element 3 to be heated are operated on the basis of information from a temperature sensor provided to the element 3 to be heated to realize fixing on the recording material by heating at appropriate temperatures.
Suppose that a printing device with the aforementioned fixing unit shown in FIG. 11 is placed in a relatively warm place in an environment of a temperature ranging from a room temperature to a temperature during summer, a recording material is placed in a well air-conditioned depository, and printing is started immediately after the recording material brought from the depository is put into the printing device. In this case, the environmental temperatures of the printing device are subjected to the correction for high temperature (temperature lowering correction) to lower a fixing temperature. Meanwhile, since the recording material gets cold sufficiently, fixing heat is not enough until the recording material adapts itself to the environmental temperatures, whereby the cold offset may occur. As a result, faulty fixing such as separation of toner may be generated.
In order for a fixing unit installed in a printing device to fix an unfixed toner image held on a recording material, the unfixed toner image may be fixed on the basis of the environmental temperatures of the printing device as shown in FIG. 11, or on the basis of the temperature of the fixing film 4 as disclosed in Japanese Patent Application Laid-open No. 2004-184621. In either case, it is not surely determined if the toner image on the recording material reaches a temperature at which fusing actually occurs. Accordingly, if a recording material holding a toner image of a temperature different from the environmental temperature of the device reaches the fixing unit, the temperature of the recording material holding the toner image may not fall within an allowable temperature range within which favorable fixing is realized. In this case, the cold offset may occur, causing fixing defects such as separation of toner.